Method of directly reading output voltage to determine data stored in a non-volatile memory cell

ABSTRACT

An NVM cell design enables direct reading of cell output voltage to determine data stored in the cell, while providing low current consumption and a simple program sequence that utilizes reverse Fowler-Nordheim tunneling.

FIELD OF THE INVENTION

The present invention relates to integrated circuit memory devices and, in particular, to a method of directly reading output voltage to determine data stored in a non-volatile memory (NVM) cell.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 7,164,606 B1, which issued on Jan. 16, 2007, to Poplevine et al., discloses an all PMOS 4-transistor non-volatile memory (NVM) cell that utilizes reverse Fowler-Nordheim tunneling for programming.

Referring to FIG. 1, as disclosed in U.S. Pat. No. 7,164,606, in accordance with the method of programming an NVM array that includes all-PMOS 4-transistor NVM cells having commonly-connected floating gates, for each cell 100 in the array that is to be programmed, all of the electrodes of the cell are grounded. Then, an inhibiting voltage V_(N) is applied to the bulk-connected source region V_(r) of the cell's read transistor P_(r), to the commonly-connected drain, bulk and source regions V_(e) of the cell's erase transistor P_(e), and to the drain region D_(r) of the read transistor P_(r). The source region V_(p) and the drain region D_(p) of the cell's programming transistor P_(w) are grounded. The bulk V_(nw) of the programming transistor P_(w) is optional; it can be grounded or it can remain at the inhibiting voltage V_(N). For all cells in the NVM array that are not selected for programming, the inhibiting voltage V_(N) is applied to the V_(r), V_(e) and D_(r) electrodes and is also applied to the V_(p), D_(p) and V_(nw) electrodes. The control gate voltage V_(c) of the cell's control transistor P_(c) is then swept from 0V to a maximum programming voltage V_(cmax) in a programming time T_(prog). The control gate voltage V_(c) is then ramped down from the maximum programming voltage V_(cmax) to 0V. All electrodes of the cell and the inhibiting voltage V_(N) are then returned to ground.

As described in detail in the '606 patent, the all-PMOS 4-transistor NVM cell disclosed therein relies on reverse Fowler-Nordheim tunneling for programming. That is, when the potential difference between the floating gate electrode of the programming transistor of the all-PMOS NVM cell and the drain, source and bulk region electrodes of the programming transistor exceeds a tunneling threshold voltage, electrons tunnel from the drain and source electrodes to the floating gate, making the floating gate negatively charged.

U.S. Pat. No. 7,164,606 is hereby incorporated by reference herein in its entirety to provide background information regarding the present invention.

The all-PMOS 4-transistor NVM cell programming technique disclosed in the '606 patent provides advantages of both low current consumption, allowing the ability to simultaneously program a large number of cells without the need for high current power sources, and a simple program sequence. To read data from the all-PMOS NVM cell 100, output current is sensed and compared either to a reference current or by using a differential cell structure. In addition, to translate the sensed current into a data output, a sense amplifier and latching circuit are usually required. The sense amplifier transforms the sensed current to a corresponding voltage level. The latching circuit stores the data output, since the sense amplifier is usually turned off after the cell's read cycle. Use of the sense amplifier circuit and the latching circuit adds significantly to the total area of the NVM cell array.

Thus, there is a need for an NVM cell of reduced size compared with that of the all-PMOS NVM cell, but that retains its advantages.

SUMMARY OF THE INVENTION

In accordance with an embodiment of the present invention, a non-volatile memory (NVM) cell includes an NMOS control transistor having commonly-connected source, drain and bulk regions to which a control voltage is applied; the gate of the control transistor is connected to a data storage node. The NVM cell also includes a PMOS erase transistor having commonly-connected source, drain and bulk regions to which an erase voltage is applied; the gate of the erase transistor is connected to the data storage node. The NVM cell also includes two transistors that act like an inverter circuit. A PMOS inverter transistor and an NMOS inverter transistor each have a drain that is connected to an output voltage node of the NVM cell. The NVM cell includes additional circuitry for delivering either a first supply voltage or a second lower supply voltage to the cell. A first NMOS transistor includes commonly-connected source and bulk regions that are grounded and a drain that is connected to the source of the PMOS inverter transistor. A first PMOS transistor has its commonly-connected source and bulk regions selectively connectable to receive the first supply voltage or the second supply voltage and a drain that is connected to the source of the PMOS inverter transistor. The gates of the first NMOS and PMOS transistors are both connected a first electrode. A second NMOS transistor includes commonly-connected source and bulk regions that are grounded and a drain that is connected to the source of the NMOS inverter transistor. A second PMOS transistor has its commonly-connected source and bulk regions selectively connectable to receive either the first supply voltage or the second supply voltage and its drain connected to the source of the NMOS inverter transistor. The gates of the second NMOS and PMOS transistors are both connected to a second electrode.

In a read condition for the above-described NVM cell, the supply voltage is set to the second lower supply voltage. The first electrode is set to 0V, thus setting the source of the PMOS inverter transistor to the second supply voltage. The second electrode is set to the second supply voltage, thus setting the source of the NMOS inverter transistor to 0V. All other electrodes of the NVM cell are set to 0V. Under this read condition, the output data of the NVM cell may be read on the cell's output voltage node.

The features and advantages of the various aspects of the present invention will be more fully understood and appreciated upon consideration of the following detailed description of the invention and the accompanying drawings, which set forth illustrative embodiments in which the concepts of the invention are utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an all-PMOS 4-transistor NVM cell.

FIG. 2 is a schematic diagram illustrating an embodiment of an NVM cell in accordance with the concepts of the present invention.

FIG. 3 is a schematic diagram illustrating an embodiment of an NVM cell array row that includes a plurality of NVM cells of the type shown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows an NVM cell 200 that enables the direct read of output voltage to determine data stored in the cell 200, while maintaining the simple program sequence and low current consumption advantages of the FIG. 1 all-PMOS 4-transistor NVM cell 100.

As shown in FIG. 2, the NVM cell 200 includes an NMOS control transistor N_(c) having commonly-connected source, drain and bulk region electrodes to which a control voltage V_(c) is applied; the gate electrode of the control transistor N_(c) is connected to a data storage node N_(s). The cell 200 also includes a PMOS erase transistor P_(e) having commonly-connected source, drain and bulk region electrodes to which an erase voltage V_(e) is applied; the gate electrode of the erase transistor V_(e) is connected to the data storage node N_(s).

The NVM cell 200 also includes two transistors, PMOS inverter transistor P_(inv) and NMOS inverter transistor N_(inv), that, as discussed in greater detail below, act like an inverter circuit. PMOS inverter transistor P_(inv) includes a source electrode S_(p), a bulk region electrode V_(nw) and a drain electrode connected to the cell's voltage output node Out. NMOS inverter transistor N_(inv) includes a source electrode S_(n), a bulk region electrode V_(pw) and a drain electrode connected to the voltage output node Out of the NVM cell 200.

The NVM cell 200 includes additional circuitry for delivering either a first supply voltage V_(N1) or a second lower supply voltage V_(N2) to the cell 200, as discussed in greater detail below. A first NMOS transistor 202 includes commonly-connected source and bulk region electrodes that are grounded and a drain electrode that is connected to the source electrode S_(p) of PMOS transistor P_(inv). A first PMOS transistor 204 has its commonly-connected source and bulk region electrodes selectively connectable to receive either the first supply voltage V_(N1) or the second lower supply voltage V_(N2) and its drain electrode connected to the source electrode S_(p) of PMOS transistor P_(inv). The gate electrode of the first NMOS transistor 202 and the gate electrode of the first PMOS transistor 204 are both connected to electrode A. A second NMOS transistor 206 includes commonly-connected source and bulk region electrodes that are grounded and a drain electrode that is connected to the source electrode S_(n) of NMOS transistor N_(inv). A second PMOS transistor 208 has its commonly-connected source electrode and bulk region electrode selectively connectable to receive either the first supply voltage V_(N1) or the second lower supply voltage V_(N2) and its drain electrode connected to the source electrode S_(n) of the of NMOS transistor N_(inv). The gate electrode of the second NMOS transistor 206 and the gate electrode of the second PMOS transistor 208 are both connected to electrode B.

FIG. 3 shows an embodiment of an NVM cell array row 300 that includes a plurality of NVM cells of the type shown in FIG. 2.

As stated above, the NVM cell 200 shown in FIG. 2 changes the structure of the all-PMOS 4-transistor NVM cell 100 shown in FIG. 1 by adding circuitry that directly translates the data stored in the NVM cell 200 into rail-to-rail output voltage. As shown in FIG. 2, this is realized by using by using two different supply voltage levels, where supply voltage level V_(N1), which is used during the erase and programming sequences is higher than supply voltage level V_(N2), which is used during a read sequence. After a program sequence, which utilizes the same reverse Fowler-Nordheim tunneling method described in the above-cited '606 patent, the difference in the voltage level between the floating gate of a programmed NVM cell 200 and the floating gate of a non-programmed NVM cell 200 is about V_(N1). Then, during a read sequence, the P_(inv) transistor and the N_(inv) transistor act like an inverter circuit, at power supply level V_(N2) and ground level 0V, with input connected to the floating gate of the NVM cell that has a voltage level that is either turning off the P_(inv) transistor completely (while turning on the N_(inv) transistor) or turning off the N_(inv) transistor completely (while turning on the P_(inv) transistor). This can be done by adjusting the maximum erase voltage V_(emax) and the maximum control voltage V_(cmax) (which are used during the erase sequence and the programming sequence, respectively) to shift the voltage of the floating gate of a programmed NVM cell 200 and the floating gate of a non-programmed NVM cell 200 by the same amount up or down, since the voltage difference V_(N1) is larger than the voltage difference V_(N2). This condition results in the voltage output electrode Out of the NVM cell 200 being driven to either V_(N2) or 0V, thus producing rail-to-rail voltage output. The N_(inv) transistor is an NMOS device with its substrate region (V_(pw) electrode) realized using either a common P-substrate (tied to 0V) or isolated P-wells tied to its source region (S_(n) electrode); the P_(inv) transistor is a PMOS device with its substrate region (V_(nw) electrode) realized using either V_(N1)/V_(N2) (common N-well) or isolated N-wells tied to its source region (S_(p) electrode).

The FIG. 2 NVM cell 200 eliminates the need to have a sense amplifier circuit or a latching circuit as part of the output circuitry of the cell. It also eliminates the need to have a reference current for comparison, which is usually generated by reference cells or by using a differential cell structure, both of which significantly increase the total area of the NVM cell array row. Thus, the FIG. 2 cell 200 simplifies the overall circuit design and reduces the size of the array row, as compared with the FIG. 1 all-PMOS NVM cell 100. At the same time, the FIG. 2 cell 200 retains the advantages of the FIG. 1 cell 100 by using the same method of programming and erasing using Fowler-Nordheim tunneling. As discussed in greater detail below, The program sequence and erase sequence are only slightly different, while the read sequence is significantly simplified (the data output is readily available from the cell NVM 200 on idle state).

Referring to FIGS. 2 and 3, a summary of the program, erase and read sequences for the FIG. 2 NVM cell 200 in an array row 300 is as follows:

Program Sequence

Setting all electrodes to 0V, with the supply voltage set to V_(N1). For all NVM cells to be programmed, setting the A electrode and the B electrode to voltage V_(N1), thus setting all S_(p), S_(n) and Out electrodes for selected cells to voltage 0V. For all cells selected to not be programmed, setting the A electrode and the B electrode to 0V, thus setting all S_(p), S_(n) and Out electrodes of non-selected cells to voltage V_(N1). Ramping up the control voltage V_(c) from 0V to a predefined maximum control voltage V_(cmax) for a preselected programming time T_(prog) while ramping up the erase voltage V_(e) from 0V to a predefined maximum erase voltage V_(emax) for the programming time T_(prog); the V_(e) electrode is ramped up along with the V_(c) electrode to prevent forward biasing the diode that is formed between the isolated P-well and the N-well. Ramping down the V_(c) electrode from V_(cmax) to 0V and the V_(e) electrode from V_(emax) to 0V. Returning all electrodes with voltage V_(N1) to 0V, thus completing the program sequence.

Erase Sequence

Setting all electrodes to 0V, with the supply voltage set to V_(N1). Setting the A electrodes and the B electrodes to voltage V_(N1), thus setting all S_(p), S_(n) and Out electrodes to voltage 0V. ramping up the erase voltage V_(e) from 0V to the maximum erase voltage V_(emax) for the duration of a preselected erase time T_(erase). At the end of the erase time T_(erase), ramping down the erase voltage V_(e) from the maximum erase voltage V_(emax) to 0V. Returning all electrodes with voltage V_(N1) to 0V, thus completing the erase sequence.

Read Condition

While the supply voltage is set to V_(N2), setting the A electrode to voltage 0V, thus setting the S_(p) electrode to voltage V_(N2), setting the B electrode to voltage V_(N2), thus setting the S_(n) electrode to 0V, and setting all other electrodes to 0V. Under this read condition, the output data of the cell 200 is on the cell's Out electrode. (It is note that, since the NVM cell in its virgin state has an unknown floating gate voltage level, both the A electrode and the B electrode should be set to voltage V_(N1) to prevent short current from flowing through the P_(inv) and N_(inv) transistors.

Those skilled in the art will appreciate that the voltage levels utilized in the program, erase and read operations will depend upon the thickness of the gate oxide utilized in the NMOS and PMOS devices of the NVM cell. For example, for agate oxide thickness of 60-80 Å, V_(N1)˜=3.3V, V_(cmax)=V_(emax)˜=10V, with T_(prog)=T_(erase)˜=20-50 milliseconds. For gate oxide thickness of 120 Å, V_(N1)˜=5.0V, V_(cmax)=V_(emax)˜=16V, with T_(prog)=T_(erase)˜=20=50 milliseconds. Supply voltage level V_(N2) should be smaller than supply voltage level V_(N1), the difference depending upon the process technology utilized.

It should be understood that the particular embodiments of the invention described above have been provided by way of example and that other modifications may occur to those skilled in the art without departing from the scope of the invention as expressed in the appended claims and their equivalents. 

1. A non-volatile memory (NVM) cell structure comprising: an NMOS control transistor having commonly-connected source, drain and bulk region electrodes to which a control voltage is applied and a gate electrode connected to a data storage node; a PMOS erase transistor having commonly-connected source, drain and bulk region electrodes to which an erase voltage is applied and a gate electrode connected to the data storage node; a PMOS inverter transistor having a drain electrode connected to an output voltage node of the NVM cell; an NMOS inverter transistor having a drain electrode connected to the output voltage node; a first NMOS transistor that includes commonly-connected source and bulk region electrodes that are grounded, a drain electrode that is connected to the source of the PMOS inverter transistor, and a gate electrode connected to a first electrode; a first PMOS transistor having commonly-connected source and bulk region electrodes selectively connectable to receive the first supply voltage or the second supply voltage, a drain electrode that is connected to the source of the PMOS inverter transistor, and a gate electrode connected to the first electrode; a second NMOS transistor that includes commonly-connected source and bulk region electrodes that are grounded, a drain electrode that is connected to the source electrode of the NMOS inverter transistor; and a gate electrode connected to a second electrode; and a second PMOS transistor having commonly-connected source and bulk region electrodes selectively connectable to receive either the first supply voltage or the second supply voltage, a drain electrode connected to the source electrode of the NMOS inverter transistor, and a gate electrode connected to the second electrode.
 2. A non-volatile memory (NVM) cell array row that includes a plurality of NVM cells, each NVM cell in the array row comprising: an NMOS control transistor having commonly-connected source, drain and bulk region electrodes to which a control voltage is applied and a gate electrode connected to a data storage node; a PMOS erase transistor having commonly-connected source, drain and bulk region electrodes to which an erase voltage is applied and a gate electrode connected to the data storage node; a PMOS inverter transistor having a drain electrode connected to an output voltage node of the NVM cell; an NMOS inverter transistor having a drain electrode connected to the output voltage node; a first NMOS transistor that includes commonly-connected source and bulk region electrodes that are grounded, a drain electrode that is connected to the source of the PMOS inverter transistor, and a gate electrode connected to a first electrode; a first PMOS transistor having commonly-connected source and bulk region electrodes selectively connectable to receive the first supply voltage or the second supply voltage, a drain electrode that is connected to the source of the PMOS inverter transistor, and a gate electrode connected to the first electrode; a second NMOS transistor that includes commonly-connected source and bulk region electrodes that are grounded, a drain electrode that is connected to the source electrode of the NMOS inverter transistor; and a gate electrode connected to a second electrode; and a second PMOS transistor having commonly-connected source and bulk region electrodes selectively connectable to receive either the first supply voltage or the second supply voltage, a drain electrode connected to the source electrode of the NMOS inverter transistor, and a gate electrode connected to the second electrode.
 3. A method of reading data from a non-volatile memory (NVM) cell wherein the NVM cell includes an NMOS control transistor having commonly-connected source, drain and bulk region electrodes to which a control voltage is applied and a gate electrode connected to a data storage node; a PMOS erase transistor having commonly-connected source, drain and bulk region electrodes to which an erase voltage is applied and a gate electrode connected to the data storage node; a PMOS inverter transistor having a drain electrode connected to an output voltage node of the NVM cell; an NMOS inverter transistor having a drain electrode connected to the output voltage node; a first NMOS transistor that includes commonly-connected source and bulk region electrodes that are grounded, a drain electrode that is connected to the source of the PMOS inverter transistor, and a gate electrode connected to a first electrode; a first PMOS transistor having commonly-connected source and bulk region electrodes selectively connectable to receive the first supply voltage or the second supply voltage, a drain electrode that is connected to the source of the PMOS inverter transistor, and a gate electrode connected to the first electrode; a second NMOS transistor that includes commonly-connected source and bulk region electrodes that are grounded, a drain electrode that is connected to the source electrode of the NMOS inverter transistor; and a gate electrode connected to a second electrode; and a second PMOS transistor having commonly-connected source and bulk region electrodes selectively connectable to receive either the first supply voltage or the second supply voltage, a drain electrode connected to the source electrode of the NMOS inverter transistor, and a gate electrode connected to the second electrode, a method comprising: setting the first electrode to the 0V, thereby setting the source electrode of the PMOS inverter transistor to the second supply voltage; setting the second electrode to the second supply voltage, thereby setting the source of the NMOS inverter transistor to 0V; setting all other electrodes of the NVM cell to 0V, whereby output data of the NVM cell is available on the output voltage node. 